Our major aim in these studies is to develop an in vitro model system for investigating various phases of respiratory carcinogenesis at the cellular level. While most of the evidence for a multistage oncogenic originates from mouse skin experiments, several recent reports using fibroblast systems suggest that the initiation-promotion process can be carried out in a well-defined in vitro situation. Several studies are now in progress as our first steps in developing an in vitro system for respiratory tract epithelium.